Bring 1 cup (2 sticks) butter to room temperature. Transfer butter, sage, lemon juice, 1/2 teaspoon salt, and garlic to the bowl of food processor. Process until pureed. Transfer mixture to a small bowl and cover; keep refrigerated for up to 5 days. Bring to room temperature before using.

Place one fish fillet on a cutting board, skin side down. Using a very sharp knife, make a small horizontal incision between the skin and the flesh of the fish, about 1 inch from the thinner end of the fillet. Slide knife toward the thin end, separating skin from flesh. Turn the knife around and cut toward the thicker end of the fish, pulling the skin and stopping about 1/2 inch from the end skin remains attached. Repeat process with remaining three fillets.

Spread 2 tablespoons of the sage butter mixture between the skin and flesh of each fillet. Wrap skin over the thin end of the fish to hold in place. Fillets can be wrapped with plastic wrap and refrigerated, skin side up, for up to 24 hours. Let come to room temperature before proceeding.

Cut up remaining 1 cup (2 sticks) butter into 1/2-inch cubes. Keep refrigerated until ready to use. Place wine, vinegar, and port in a large, heavy-bottomed, nonreactive saucepan over high heat. Add shallots and bring to a boil. Reduce heat to a simmer and cook, brushing down the sides of the saucepan with a clean, dry pastry brush, until syrupy and reduced to about 2 tablespoons, about 20 minutes.

Reduce heat to low and gradually whisk in cold butter, a few cubes at a time, whisking until well blended before each addition; season with salt and pepper. Set a fine-mesh sieve over a small saucepan and strain sauce into saucepan; keep warm, whisking occasionally.

Place Wondra flour in a shallow dish. Dredge fish fillets in flour and season with salt and pepper; set aside.